Extended Data Fig. 5: Photostimulation evoked responses on single trials.

a-c, Comparison of the amplitude of photostimulation evoked response of the target neuron and causally connected neurons, shown for 3 example neuronal pairs. Trials were categorized according to the strength of the evoked response (Δ Activity) of the target neurons, into trials with ‘strong’ or ‘weak’ response of the target (Methods). Responses of both target and causally connected neurons are shown by trial-averaging the activity for strong (red), weak (blue), or all (black) trials, displayed as mean ± s.e.m. across trials. The number of trials in each category for example neuronal pairs: (a) 15 strong, 15 weak, 75 all; (b) 13 strong, 13 weak, 63 all; (c) 15 strong, 15 weak, 75 all. d, Left, Distribution of Pearson correlation (r) between Δ Activity of target neurons and causally connected neurons on a trial-by-trial basis, shown for all neuronal pairs with significant causal connections (n = 33,830 connection pairs; Methods). To control for shared network activity that could lead to spurious evoked response correlations, we recomputed correlations after projecting out the first principal component of neural activity (middle) or the first three principal components (right). While the magnitude of correlations decreased, positive correlations persisted (average correlations r = 0.23 for full activity, r = 0.18 after removing the first PC, and r = 0.16 after removing the first three PCs), suggesting that the covariation in activity reflects functional interactions rather than spontaneous network fluctuations.